Threshold Limit Values are a useful aid in controlling the air quality in industrial work areas. However, these values are established for individual pollutants, whereas the workman is most often exposed to mixtures of agents. In these situations the effects of the different agents may be more or less independent of each other. This study is intended to quantitate the biological response of guinea pigs exposed to an atmosphere containing HF at the TLV, to particulate fluoride (insoluble rock phosphate or soluble triple superphosphate) at the TLV, and to mixtures of the gaseous and particulate fluorides at their respective TLVs. Studies will also be done with mixtures where in the concentrations of these components will be reduced to conform to the calculated TLV for a mixture of these two agents. The results are expected to indicate not only the biological effects of airborne gaseous of particulate fluorides at the TLVs of these agents, but also the effects of concurrent exposures to the two agents. The experiments will indicate the extent to which the effects of these two components are additive, and will give some indication of the margin of safety associated with the individual TLVs and with the TLV of the mixture. The agents chosen are among those encountered in the mining and processing or rock phosphate to produce phosphoric acid and phosphate fertilizers.